The Guardian in Baldur's Gate 3 is a character that you'll have to design during character creation. You won't then see or hear from them for hours once you start playing.

As you continue to play Baldur's Gate 3, you'll eventually learn in the second and third acts that The Guardian is actually The Emperor, a person who was turned into a Mind Flayer while adventuring.

I've met my guardian, but I have no idea what their ultimate significance is, because I'm still in Act 1, which was already available in the early access version of Baldur's Gate 3.